This invention relates to a novel sensitive article for electrophotography which comprises an electric charge generating substance and an electric charge transfer substance. More particularly, this invention relates to a novel sensitive article for electrophotography which uses a novel polyester as an active component for the electron charge transfer substance.
Heretofore, selenium, cadmium sulfide, zinc sulfide, a-silicon, and selenium-tellurium have been proposed as photoconductive materials for sensitizing articles for electrophotography.
The electrophotography is a process for forming a visible image of given graphic matter by electrifying a sensitive article in a dark place, then exposing the sensitive article to light through the graphic matter thereby selectively discharging electricity from the sensitive article and forming a latent image, and subsequently developing the latent image portion of the sensitive article with a toner. The sensitive article to be used in the electrophotography is required to possess high capacity for electrification in the dark place, admit of only sparing discharge or dark current, and provide quick release of electric charge upon irradiation with light or, in other words, enjoy high sensitivity. As photoconductive materials capable of satisfying all these requirements, numerous inorganic photoconductive materials such as are enumerated above have been accepted for actual use.
In contrast to those inorganic sensitive articles, there have been proposed many electrophotographic sensitive articles which use organic photoconductive materials featuring non pollution, good processability, high flexibility, and light weight.
Owing to the discovery that organic photoconductive materials each formed of a combination of an electric charge generating substance and an electric charge transfer substance possess outstanding properties as sensitive materials, electrophotographic sensitive articles using various organic substances have been proposed. For example, electrophotographic sensitive articles using a combination of poly-N-vinylcarbazole with 2,4,7-trinitro-9-fluorenone (U.S. Pat. No. 3,484,237), a combination of a pyrazoline compound with chlorodianeblue or squanylium (Japanese Patent Application Laid-Open No. 55643/77 and No. 105536/74), a combination having thiapyrylium salt and triphenylmethane dispersed in polycarbonate resin [J. Appl. Phys., 49, 5543 (1978)], and a combination of 9-(4-diethylaminostyryl) anthracene or N-methyl-N-phenylhydrazo-3-methylidene-9-ethylcarbazole with a bis-azo compound (Richo. Tech. Report., 1980(3), 4), etc. have been proposed. Researches are now actively under way as aimed at developing electric charge generating substances and electric charge transfer substances of increasingly high efficiency, completing sensitive structures by optimum combination of such newly developed substances, and improving practical properties of such sensitive articles. As electric charge transfer substances, there have been proposed pyrazoline compounds, 9-(4-diethylaminostyryl) anthracene, N-methyl-N-phenylhydrazo-3-methylidene-9-ethylcarbazole, etc. which are invariably low molecular compounds.
These compounds, however, still have many defects. For example, these compounds have no sufficient stability in their independent form and, for use in a sensitive article, inevitably necessitate additional use of a polymeric binding agent because they are incapable of forming a film unless they are dissolved or dispersed in the binding agent. Since the sensitive article relies for transfer of electric charge upon such an electric charge transfer substance, it is desired to contain the electric charge transfer substance amply. Thus, there is a possibility that the greater part of the sensitive article is accounted for by a low molecular-weight electric charge transfer substance. Consequently, the compatibility of the electric charge transfer substance in the polymeric substance being used as the binding agent, the transparency of the film consequently formed of the electric charge transfer substance dispersed in the binding agent, and the properties of the produced film may pose themselves as problems. The selection of the polymeric binding agent to be additionally used as described above, therefore, demands especial care. When there is adopted a low molecular-weight electric charge transfer substance which by nature lacks compatibility in a polymeric binding agent, the selection of a polymeric substance and the formulation of the electric charge transfer substance with the selected polymeric substance are required to be carried out most attentively lest the transparency and other physical properties of the produced film should be degraded. Despite the great care thus taken, since the low molecular-weight compound in a large amount is dissolved or dispersed in the form of filler in the polymeric substance, the film or coat consequently produced from the resultant combination no longer retains the outstanding flexibility, thermal resistance, and strength possessed inherently by the polymeric substance.
For the elimination of these defects, high molecular-weight electric charge transfer substance is desired to be capable of being converted in its unmodified form into a film or coat.
Because most of the conventional polymeric electric charge transfer substances are not easily mass produced or because they are incapable of being polymerized to sufficiently high molecular weights, only a very few polymeric electric charge transfer substances such as poly-N-vinylcarbazole and halogenated poly-N-vinylcarbazole have been so far demonstrated to fulfil the requirement and prove feasible.
The inventors of the present invention carried out a devoted study in search for a high molecular-weight substance effectively functioning as an electric charge transfer substance for electrophotographic sensitive article and possessing an ample film-forming property. They have consequently found that a polyester obtained from 2,6-dimethoxy-9,10-anthracene diol and an .alpha.,.omega.-dicarboxylic acid constitutes itself an excellent high molecular-weight electric charge transfer substance and effectively functions as an electric charge transfer substance for an electrophotographic sensitive article. The present invention has issued from this discovery.
The inventors formerly found that a polyester obtained from an anthracene diol type compound and a dicarboxylic acid is useful as an organic fluorescent substance or organic semiconductor (Japanese Patent Application No. 172546/80) and also proposed a method for commercial manufacture of this polyester (Japanese Patent Application No. 174717/80). After a further study, the inventors of the present invention have ascertained that a polyester obtained from 2,6-dimethoxy-9,10-anthracene diol and an .alpha.,.omega.-dicarboxylic acid manifests specifically outstanding properties when it is used as an electric charge transfer substance for an electrophotographic sensitive article.
As 9,10-anthracene diol derivatives, various substitution products such as, for example, 2-methoxy-9,10-anthracene diol, 2,3-dimethoxy-9,10-anthracene diol, and 2,7-isopropoxy-9,10-anthracene diol have also been conceived. A comparative study conducted on all these substitution products has revealed that the 2-, 2,3-, and 2,7-substitution products are invariably inferior in properties to the 2,6-dimethoxy substitution product, irrespectively of the kind of substituents involved therein. The reason for their inferiority is still unknown at present. When polyesters are synthesized from these diols and .alpha.,.omega.-dicarboxylic acids, the 2,6-dimethoxy substitution products exhibit high crystallinity and the 2-, 2,3-, and 2,7-substitution products exhibit only low crystallinity. This lower crystallinity may possibly explain why the latter substitution products fail to manifest an ample electric charge transfer activity. Among other substitution products, those using a methoxy group impart the highest levels of sensitivity to the sensitive articles obtained from the corresponding polyesters. A possible reason for the excellent sensitivity may be that the incorporation of a methoxy group will lower the ionization potential and, at the same time, enhance the crystallinity of the polyester of the present invention.